How to Build a Dinosaur Den

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Oryctodromeus isn't exactly a household name. A small, herbivorous ornithopod found in the Late Cretaceous rock of western North America, it was the sort of dinosaur most often depicted as being prey for charismatic carnivores. But there was at least one aspect of Oryctodromeus that made it particularly interesting—this dinosaur may have lived in burrows.

Based on the context of the rocks they are found in, we know that dinosaur bodies were preserved in a variety of different environments. Some bodies were covered up by seasonal floods, other dinosaurs were washed out to sea, and dinosaurs even died in death-traps created by the footprints of even bigger species. But until Oryctodromeus, dinosaurs had not been found in fossilized dens.

The fact that the small dinosaurs had been buried within a burrow was made clear by the details of their den. At the end of an S-shaped tunnel was a large chamber that had been dug into three different layers of mudrock and later filled with sandstone. The fact that an adult and two juvenile Oryctodromeus were found in the sandstone confirmed that this was a den that had been flooded by a slurry of water and sandy sediment.

But were the dinosaurs buried inside their den, or had their bodies just been washed inside? The dinosaur bones were jumbled up rather than lying in articulated poses on the burrow floor. This left the details of their preservation unclear. In order to solve this mystery, paleontologists Cary Woodruff and David Varricchio created a half-scale model of the original burrow with PVC pipes and conducted experiments with rabbits to see what sort of scenario would best account for the way the dinosaur fossils had been preserved.

The paleontologists ran thirteen trials by filling their artificial burrow with a mixture of water, clay, and sand. Rather than using whole rabbits, though, Woodruff and Varricchio only used disarticulated skeletons. This is because no Oryctodromeus bones were found in their natural positions, hinting that the dinosaurs died, decomposed, and had mostly fallen apart before their preservation. By the time the den was flooded, the dinosaurs had already turned into piles of bones (regardless of whether their skeletons were inside or outside the burrow at the time of the event).

Woodruff and Varricchio modeled the different ways the bones could have found their way into the den by running a variety of tests. In some trials the bones were placed in the burrow, while in others they were included in the sediment mix used to fill the artificial den. Each setup produced a different distribution of bones in the PVC chamber.

Six different trials with differing conditions all created the kind of elevated, dispersed assemblage of bones found in the Oryctodromeus burrow. Bones were initially inside the chamber for four of these trials, but were outside the burrow and contained within the sediment, respectively, in the other two. While this evidence supports the idea that the dinosaur bones may have been inside the den when it was flooded, it remains possible that the bones were washed in from outside.

If the dinosaur skeletons really were washed into the burrow from outside, however, Woodruff and Varricchio argue, it is strange that the bones of an adult and two juveniles should be found together. Furthermore, bones transported by sediment-filled floods are often broken and abraded, and there are no signs of such destructive transport on the Oryctodromeus fossils. The hypothesis that the Oryctodromeus bones were already inside the den remains the best-supported idea. Woodruff and Varricchio caution that further investigations are required to understand how these dinosaurs—and other den-dwelling fossil vertebrates—became preserved.



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